From 45a0b812fa13ec255cae91f974540a4d805a8d79 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Mon, 20 Feb 2023 09:46:54 +0100
Subject: [PATCH] [InstCombine] Remove early constant fold

InstCombine currently performs a constant folding attempt as part
of the main InstCombine loop, before visiting the instruction.
However, each visit method will also attempt to simplify the
instruction, which will in turn constant fold it. (Additionally,
we also constant fold instructions before the main InstCombine loop
and use a constant folding IR builder, so this is doubly redundant.)

There is one place where InstCombine visit methods currently don't
call into simplification, and that's casts. To be conservative,
I've added an explicit constant folding call there (though it has
no impact on tests).

This makes for a mild compile-time improvement and in particular
mitigates the compile-time regression from enabling load
simplification in be88b5814d9efce131dbc0c8e288907e2e6c89be.

Differential Revision: https://reviews.llvm.org/D144369
---
 llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp    |  4 ++++
 .../lib/Transforms/InstCombine/InstructionCombining.cpp | 17 -----------------
 llvm/test/Transforms/InstCombine/pr38677.ll             |  3 +--
 3 files changed, 5 insertions(+), 19 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 71fe7fd..d723b8b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -308,6 +308,10 @@ Instruction *InstCombinerImpl::commonCastTransforms(CastInst &CI) {
   Value *Src = CI.getOperand(0);
   Type *Ty = CI.getType();
 
+  if (auto *SrcC = dyn_cast<Constant>(Src))
+    if (Constant *Res = ConstantFoldCastOperand(CI.getOpcode(), SrcC, Ty, DL))
+      return replaceInstUsesWith(CI, Res);
+
   // Try to eliminate a cast of a cast.
   if (auto *CSrc = dyn_cast<CastInst>(Src)) {   // A->B->C cast
     if (Instruction::CastOps NewOpc = isEliminableCastPair(CSrc, &CI)) {
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index d09a246..2679e0a 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -4221,23 +4221,6 @@ bool InstCombinerImpl::run() {
     if (!DebugCounter::shouldExecute(VisitCounter))
       continue;
 
-    // Instruction isn't dead, see if we can constant propagate it.
-    if (!I->use_empty() &&
-        (I->getNumOperands() == 0 || isa<Constant>(I->getOperand(0)))) {
-      if (Constant *C = ConstantFoldInstruction(I, DL, &TLI)) {
-        LLVM_DEBUG(dbgs() << "IC: ConstFold to: " << *C << " from: " << *I
-                          << '\n');
-
-        // Add operands to the worklist.
-        replaceInstUsesWith(*I, C);
-        ++NumConstProp;
-        if (isInstructionTriviallyDead(I, &TLI))
-          eraseInstFromFunction(*I);
-        MadeIRChange = true;
-        continue;
-      }
-    }
-
     // See if we can trivially sink this instruction to its user if we can
     // prove that the successor is not executed more frequently than our block.
     // Return the UserBlock if successful.
diff --git a/llvm/test/Transforms/InstCombine/pr38677.ll b/llvm/test/Transforms/InstCombine/pr38677.ll
index bacee2c..f60c7fb 100644
--- a/llvm/test/Transforms/InstCombine/pr38677.ll
+++ b/llvm/test/Transforms/InstCombine/pr38677.ll
@@ -11,9 +11,8 @@ define i32 @foo(i1 %which, ptr %dst) {
 ; CHECK:       delay:
 ; CHECK-NEXT:    br label [[FINAL]]
 ; CHECK:       final:
-; CHECK-NEXT:    [[USE2:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ select (i1 icmp eq (ptr @A, ptr @B), i32 2, i32 1), [[DELAY]] ]
 ; CHECK-NEXT:    store i1 false, ptr [[DST:%.*]], align 1
-; CHECK-NEXT:    ret i32 [[USE2]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   br i1 true, label %final, label %delay
-- 
2.7.4